1. Field of the Invention
The present invention relates to a silver halide photographic emulsion which is spectrally sensitized with a supersensitizing combination of at least two kinds of sensitizing dyes.
2. Description of the Prior Art
Certain cyanine dyes are well known to increase very effectively the sensitivity of a silver halide photographic emulsion.
In such a case, high sensitivity in a longer wavelength region than the spectrally sensitive wavelength region inherent to the silver halide can be additionally imparted to a silver halide photographic emulsion by adsorption of cyanine dyes added thereto on the silver halide particles contained in the silver halide photographic emulsion. On the other hand, it is known that a certain second dye or other organic compounds, peculiarly selected with respect to the cyanine dye which was added first to the silver halide photographic emulsion, may markedly increase the efficiency of dye sensitization and, particularly, specific combinations of certain dyes provide high sensitivity to such an extent that more sensitivity than that attributed to the respective dyes is achieved. This effect is called as "supersensitization". Since no increase or decrease in the efficiency of sensitization has been conventionally experienced in the combined use of two or more dyes, supersensitization is said to be a superior effect.
Some combinations of sensitizing dyes showing supersensitization have been reported with the intention of increasing the sensitivity of a silver halide photographic emulsion. However, many of these known supersensitizing combinations are inadequate for silver halide color photographic sensitive materials because they reduce the sensitivity, cannot provide a suitable spectral sensitization distribution, cause fog and residual color, decrease the photosensitive property in the presence of other additives, or further affect adversely the stability properties after emulsion-coating. The improvement upon the deterioration with the lapse of time under high temperature and humidity conditions, i.e., an increase in fog, a decrease in sensitivity or the like, has been one of the important subjects in the art of sensitization.